Demolition marks start of Three Gorges dam flood role

On Tuesday it took 192 tonnes of dynamite and 2540 detonators to demolish a temporary structure that had protected China's massive Three Gorges Dam during construction.

The demolition marks a significant milestone for the world's largest and most controversial dam, which now starts its flood control role two years ahead of schedule.

It took place at 1600 local time (0900 BST) and lasted only 12 seconds. Just under a thousand consecutive explosions destroyed the cofferdam, which has been generating power for construction crews and holding back the new reservoir since it was filled in 2003. The cofferdam was 580 metres long and 140 metres high.

The explosive power used would have been enough to topple 400 10-storey buildings, a senior demolition project designer told China's Xinhua official state news agency.

The main dam was protected from the explosions using airbag-like underwater "bubble curtains", steel cages and mountains of sandbags. Xinhua reported that these measures would deflect up to 70% of the blast energy. You can see a video of the dam blowing in this BBC news story.

"Fantastic engineering"

The removal of the cofferdam shows that the construction of this "fantastic piece of engineering" is very advanced, says Andy Hughes, vice president of the International Commission on Large Dams, a non-governmental dam engineering advisory group based in Paris, France.

"The cofferdam itself is major structure, and there's a fine balance between how much you spend on it and how much protection you give the main dam during construction," he told New Scientist.

The Three Gorges Dam - the main structure of which was completed on 20 May - is the world's largest. At 2.3 kilometres long and 185 metres high, it is five times larger than the Hoover Dam on the border between Arizona and Nevada in the US.

Its 26 turbines will generate as much power as 18 coal-burning power stations. It was originally predicted to meet 10% of China's total energy needs when all turbines are fully operational in 2008. But due to surging demand, it is now predicted to meet just 3%.

Flood protection

However, flood control rather than energy generation is the dam's most important function. According to the China Three Gorges Project Corporation, the dam will protect 15 million people and 1.5 million hectares of farmland in the middle and lower regions of the Yangtze River from floods of a severity experienced only once every century.

Over 300,000 people have been killed in the five biggest Yangtze floods since 1931. But the new flood protection has come at significant cost.

The official cost of the dam is $22.5 billion, though that figure is much disputed by the project's critics. Furthermore, over a million people have been relocated and a thousand or more towns and villages will disappear under the 660-kilometre-long reservoir when it reaches its full height.

Pressure groups argue that the dam is having a colossal social and environmental impact. Sebastien Godinot of Friends of the Earth in Paris, France, claims the real figures are that 2 million people and over 4500 villages have been displaced in an operation beset with human rights abuses and "endemic corruption" over the distribution of compensation.

"The reservoir will also engulf 162 archaeological sites, some of them unique in the world," says Godinot. The dam has resulted in the widespread destruction of ecosystems upstream and downstream, he adds.

Eroding wetlands

A study published in April showed that the dam is already having a significant environmental impact. Researchers found that a 30% reduction in the sediment flowing downstream is eroding tidal wetlands, and limiting Shanghai's scope for expansion. The reduction in sediment and fresh water could also threaten vast fisheries in the East China Sea.

Hughes concedes that there have been significant disadvantages "as there would be with any project on this scale", but he believes the Chinese government has made a valiant effort to adequately re-house those displaced. "Hopefully the future benefits will far outweigh these disbenefits," he says.

The water level in the reservoir is currently at around 139 metres above sea level, but is expected to have reached 156 metres by the end of China's flood season from June to August.

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The explosive power used would have been enough to topple 400 10-storey buildings, according to officials (Image: Reuters)